1. Field of the Invention
The present invention relates generally to display apparatus, and more particularly to display apparatus for displaying still pictures on a liquid crystal panel used in word processors, personal computers and the like.
2. Description of the Background Art
Liquid crystal panels have been widely used for display portions of word processors, personal computers and the like. A liquid crystal panel is comprised of two transparent substrates having transparent electrodes therein with liquid crystals filled therebetween. A display apparatus for representing a picture on a liquid crystal panel has two types, one of which is a static type in which a voltage is applied to each pixel from a corresponding driving circuit which is provided for each display pixel of the liquid crystal panel, and the other is a simple matrix type in which a voltage is applied in a time divisional manner to each pixel of the liquid crystal panel from a common driving circuit. The later is widely used when the number of liquid crystal panels is large, because a small number of driving circuits are required for the number of display pixels.
FIGS. 1A and FIG. 1B are a schematic diagram of a liquid crystal panel of a display apparatus of a simple matrix type and a plan view showing an arrangement of a transparent electrode thereof, respectively.
Referring to FIG. 1A and FIG. 1B, in the liquid crystal panel of the simple matrix type display apparatus, a liquid crystal layer LC is interposed between a plurality of scanning electrodes C and a plurality of data electrodes S arranged in a direction so as to intersect with each other between two transparent substrates b1 and b2. In representing a picture, a selecting potential is sequentially applied to the scanning electrodes C with a potential corresponding to display data of a single row being applied to the data electrodes S. That is, the potential corresponding to the display data and the selecting potential are applied in a time divisional manner to each pixel corresponding to an intersection of a scanning electrode C and a data electrode S.
Application of a voltage above a certain threshold causes liquid crystal to enter a completely on state, where the light is passed through, and the application of a voltage below the threshold causes the liquid crystal to enter a completely off state where the light is not passed through. Accordingly, when a voltage corresponding to a difference between the potential corresponding to the display data and the selecting potential exceeds the threshold, the pixel enters an on-state, that is, it is driven to be on, and when the voltage corresponding to the difference is below the threshold, the pixel enters an off-state, that is, it is driven to be off. Basically, the simple matrix type display apparatus represents a picture on the liquid crystal panel by means of binary display indicating bright and dark by driving each pixel in a time divisional manner.
However, it is difficult to precisely reproduce subtle shades of the picture by the binary display which indicates no intermediate clarity. Therefore, it has been proposed to perform an intermediate tone display for displaying several intermediate levels of clarity with a simple matrix type display apparatus.
As one system for the intermediate tone display of a still picture, a data thinning-out system is known in which the ratio of an on-drive period of a pixel to an off-drive period in a predetermined period is changed according to the clarity to be displayed by the pixel. More specifically, a predetermined period of a plurality of frames, (tone representing cycle) is referred to as one cycle. Either of a potential corresponding to a display (bright) or a potential corresponding to a non-display (dark) is selectively applied to the data electrode every other frame period, such that among the plurality of frames constituting one cycle, the total number of frames in which a pixel is to be driven to be on, corresponds to a tone (clarity) to be displayed by the pixel. More specifically, the higher (lighter) the level of the tone displayed by the pixel, the higher the total number of frames of the cycle the pixel is to be driven on.
In the above-described data thinning-out system, for example, if an 8-frame period is determined as a tone representing cycle, a maximum of 9 tones can be represented.
FIG. 2 is a timing chart showing each voltage waveform applied to an arbitrary pixel when a tone pattern (a sequence of on-drive and off-drive in 8-frame period) is "1, 0, 0, 1, 1, 0, 0, 1" (wherein the on-drive is represented as "1" and the off-drive as "0") in a conventional display apparatus for displaying the above described tone representation. FIG. 2A shows a tone pattern, FIG. 2B shows a sequence of frames, FIG. 2C shows a waveform of a voltage applied to the data electrode, that is, a segment waveform, FIG. 2D shows a waveform of a voltage applied to the scanning electrode, that is, a common waveform, and FIG. 2E shows a waveform of an effective voltage applied to a pixel formed of a liquid crystal layer interposed between the data electrode and the scanning electrode, that is, a segment-common voltage waveform.
Referring to FIGS. 2C and 2D, the number of duties of voltages applied to the data electrode and the scanning electrode in one frame period is four, the voltage applied to the scanning electrode will be represented as the selecting potential plus V3 or minus V3 only for a 1/4 frame period in one frame period. Polarities of the segment waveform and the common waveform are inverted every other frame.
Now referring to FIG. 2C, while the segment waveform in the on-drive frame period includes portions exceeding a predetermined potential plus or minus V1 (a portion of a potential +V2), the segment waveform of the off-drive frame period includes no portion exceeding a predetermined potential plus or minus V1 (shown by a broken line). Due to a difference between a segment waveform of an on-drive frame period and a segment waveform of an off-drive frame period, while a portion A can be obtained in an on-drive frame period, which is above a threshold voltage .+-.Vth for driving a pixel to be on, the portion A which is above the threshold voltage .+-.Vth can not be obtained in an off-drive frame period, although a portion B which is approximate to the threshold voltage .+-.Vth can be obtained in an off-drive frame period (see FIG. 2E). In addition, since the polarities of the segment waveform and the common waveform are inverted every other frame as described above, a polarity of the segment-common voltage waveform is also inverted as shown in FIG. 2E, thereby alternatively driving a pixel.
In this case, of the 8-frame period, a portion A exceeding the threshold voltage .+-.Vth can be obtained in the segment-common voltage waveform in four frames. More specifically, since the pixel is driven to be on only four times during the 8-frame period, luminance of the pixel becomes visually dark, compared with a pixel driven to be on through a shorter time interval of each of the 8 frame periods (in case a tone pattern is "1, 1, 1, 1, 1, 1, 1, 1"), so that allied tone representation is performed. However, a tone pattern in case a pixel is driven to be on four times during the 8 frame period is not limited to this.
FIG. 3A to FIG. 3E are time charts showing each voltage waveform applied to an arbitrary pixel for representing the same tone as that of the above described example, the pixel is driven to be on only for four-frame period out of the 8 frame period with a tone pattern of "1, 0, 1, 0, 1, 0, 1, 0". FIG. 3A shows a tone pattern, FIG. 3B shows a sequence of frames, FIG. 3C shows a segment waveform, FIG. 3D shows a common waveform and FIG. 3E shows a segment-common voltage waveform.
In case the tone pattern is "1, 0, 0, 1, 1, 0, 0, 1" as shown in FIG. 2A to FIG. 2E, if a tone representing cycle is repeated, the pixel is driven to be on (or off) in succession every two frame periods. On the other hand, since the polarities of the segment waveform and the common waveform are inverted every other frame period, a segment-common voltage waveform in each of two frame periods T.sub.8n+4 and T.sub.8n+5 (in case of off-drive, T.sub.8n+2 and T.sub.8n+3, T.sub.8n+6 and T.sub.8n+7) in which the pixel is driven to be on (or off) in succession is completely symmetrical with respect to a ground potential GND (n=0, 1, 2 . . .). In addition, a segment waveform and a common waveform in each of a first frame period T.sub.8n+1 and a last frame period T.sub.8(n+1) in which a pixel is driven to be on are completely symmetrical with respect to the ground potential GND, and each of their polarities is opposite to each polarity in frames before and after the 8-frame period (not shown). Accordingly, the segment-common voltage waveform in each of the first frame period T.sub.8n+1 and the last-frame period T.sub.8(n+1) and the segment-common voltage waveforms in their adjacent frame periods which is not shown, are completely symmetrical with respect to the ground potential GND. As a result, a positive polarity portion and a negative polarity portion of the segment-common voltage completely cancel to each other every 8-frame period and 2-frame period. Accordingly, a mean value of the segment-common voltage applied to the pixel as an effective voltage always becomes 0 every 2-frame period and 8-frame period, so that when a tone is continuously represented in such a tone pattern, no direct voltage is applied to the liquid crystal layer forming a pixel.
However, in case a tone pattern is "1, 0, 1, 0, 1, 0, 1, 0" as shown in FIG. 3A to FIG. 3E, if the tone representing cycle is repeated, the on-drive and the off-drive are alternately performed every other frame period. Accordingly, segment-common voltage waveforms in respective two adjacent frame periods include a portion A above the threshold .+-.Vth and a portion B below the threshold .+-.Vth, respectively. Therefore, even though a polarity of the effective voltage applied is inverted every other frame, the segment-common voltage waveforms in any of the respective two adjacent frame periods of the 8-frame cycle are not symmetrical with respect to the ground potential GND. More specifically, in this case, a positive polarity portion and a negative polarity portion of a segment-common voltage do not completely cancel to each other in any period in which the tone representing cycle is repeated. As a result, a mean value of the segment-common voltage does not become 0 in any period in which the tone represented period is repeated, so that if displaying the tone is continued in such a tone pattern, a direct voltage continuously applied to a liquid crystal layer forming a pixel.
As can be seen from the foregoing, the direct voltage is continuously applied to the liquid crystal layer forming the pixel, depending on a setting of a tone pattern.
It is known that if the direct voltage is continuously applied to the liquid crystal layer forming the pixel, electric charges are stored in the electrodes having the liquid crystal provided therebetween, so that polarization occurs in the liquid crystal, which causes the liquid crystal to non-reversibly change to a certain state wherein an arrangement state of the liquid crystal molecules is determined by the applied direct voltage. If the liquid crystal forming the pixel enters such a state, an arrangement of the liquid crystal molecules does not correspond to the applied effective voltage, so that the picture becomes yellowish, whereby a picture to be represented correctly is no longer represented. Therefore, continuous application of a direct voltage to a liquid crystal forming a pixel adversely affects a liquid crystal layer, which causes deterioration of display performance of a liquid crystal panel.
In order to avoid a successive application of the direct voltage to the pixel, a tone pattern should be limited. However, in a tone display according to the above described data thinning-out system, a pixel is driven to be on/off according to a tone pattern. Accordingly, pixels other than those having tone patterns of "1, 1, 1, 1, 1, 1, 1, 1" and "0, 0, 0, 0, 0, 0, 0, 0" are turned on and off repeatedly at short time intervals according to a tone pattern. Therefore, if the interval of turning on and off is long, the turning on and off of the pixels are visible to the naked eye, resulting in a flickering phenomenon on a picture plane. .In order to prevent such a flickering on the picture plane as much as possible, an interval of a turning on and off of a pixel is preferably short.
For example, in case of displaying the same tone, the flickering on the picture plane can be more easily prevented the using the tone pattern of "1, 0, 1, 0, 1, 0, 1, 0" shown in FIG. 3A to FIG. 3E rather than by the display using the tone pattern "1, 0, 0, 1, 1, 0, 0, 1" shown in FIG. 2A to FIG. 2E. However, the display using the former tone pattern allows a direct voltage to be applied to a liquid crystal.
As is clear from the foregoing, in a conventional tone display of the data thinning-out system, a direct voltage was applied to a pixel, depending on a setting of a tone pattern, so that it was difficult to prevent both a flickering on the picture plane due to a turning on and off of the pixel, and an application of a direct voltage to the pixel. More specifically, in selecting a tone pattern, any tone pattern is eliminated that causes a direct voltage to be applied to a liquid crystal, a range of selecting tone patterns becomes narrow, thereby making it difficult to minimize flickering on a picture frame. Therefore in a display apparatus for displaying a tone by a conventional data thinning-out system, a tone pattern is selected taking into account of deteriorated quality of the picture due to both the application of the direct voltage to the liquid crystal and the flickering on picture frame caused by turning on and off a pixel. More specifically, in a tone display of the data thinning-out system, a tone pattern is selected such that a compromising point can be found between suppression of the flickering on the picture frame and prevention of application of the direct voltage to the liquid crystal as much as possible. As a result, according to the tone display of the data thinning-out system performed by a conventional display apparatus, there was a limit to an improvement of a picture quality.